Just Sharing This 2.2mtr Elite / Precision resurection... Part 2.

[moonbase]

...So .... using an adjustable angle finder set to 143.3°...

John,

Did you use an angle finder accurate to tenths of a degree or accurate to one degree? If you set it to 143.3 degrees, I assume it was a model accurate to one tenth of a degree?

There seems to be quite a choice for these gizmos, ranging from cheapies to quite expensive jobbies.


Rgds

 

[John]

John,

Did you use an angle finder accurate to tenths of a degree or accurate to one degree? If you set it to 143.3 degrees, I assume it was a model accurate to one tenth of a degree?

There seems to be quite a choice for these gizmos, ranging from cheapies to quite expensive jobbies.


Rgds

Yes it was only a cheapo thing MB, around £6 or so, that only read out to 145.3° ..**Note-- i have typed the wrong calculated angle in post #50, i have put 143.3°-- it should have said 145.3°.

 

[moonbase]

Yes it was only a cheapo thing MB, around £6 or so, that only read out to 145.3° ..**Note-- i have typed the wrong calculated angle in post #50, i have put 143.3°-- it should have said 145.3°.

Thanks, I see that screwfix and a few other similar suppliers stock them, I will have a mosey over there later today.

Rgds

 

[John]

Apologies, a bit late in replying, I have not worked with the Cband models for years but the same should apply at all frequencies.


The earliest Precision feeds for Ku band do not have a scalar ring as such, but a single 'lip' that is formed from the metal part used for affixing the three feedarms.
There was a second version produced a few years later which increased the number of surrounding rings to three and this coincided with a different design of the 90cm reflector that was decidedly thinner and did not have the fully rounded edge.

I did install three 90 cm versions on the Gorizont satellite at 14 (or was it 11 ?)West for client wishing to receive the state Russian channel in SECAM back in the very late 80's and tests confirmed at the shop that hand-holding the Cband LNB at the dish without the scalar ring included in the kit would reduce the sparklies, but I did not have a method to secure it in place without making something bespoke.
This was put down to the additional gain owing to less shadowing but I was never truly convinced because of the Ku band feed designs offered at the time.

I will try to find an old catalogue showing what Precision supplied for use at Cband but I would hazard a guess their reflectors were designed with a surface accuracy that means the improvements of signal collection using a scalar are not necessary.

interesting stuff CH, iv'e just thrown a quick sketch together to show how everything seemed to fit the calculations and physical dimensions of the component parts to make me think that it was working ok without the aid of a scalar.

 

[John]

Sounds good John, I presumed you had done this previous to your other post, anyway your now on the right track, will look forward to your reports, when time permits could you track back to 53E for signal comparison, just out of interest please.

I'm back at 53° East for a short while BM, any freq in particular you had in mind

 

[John]

I'm back at 53° East for a short while BM, any freq in particular you had in mind

Here's scan pics of 53°East, L&R. The brickwork in the pic is masking about 175mm in of the left hand side of the reflector. The 1.8 CM had a clear view of this position.
Not many hits for me on this bird in North Notts.


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[blademedia]

I'm back at 53° East for a short while BM, any freq in particular you had in mind

Yes J, I was interested in what you get from 3675 now you have the scalar in the correct position.

Using 53° East- 3675 RHC using either scalar gave me 13.6dB C/N on the Promax which was about a 3dB increase or so from what was obtained off the 1.8 CM. Completely removing the scalar, as in the pic, the C/N went up to 15.1 dB.

 

[Channel Hopper]

Here are some images of the earliest Precision Ku feed I have here.

 

[Channel Hopper]

interesting stuff CH, iv'e just thrown a quick sketch together to show how everything seemed to fit the calculations and physical dimensions of the component parts to make me think that it was working ok without the aid of a scalar.

Feed design is one of the more expensive outlays in satellite technology, many manufacturers simply do not bother and rely on third party companies to provide a solution, which is why Chaparral and others have a range of settings on their products. The scalar ring in these cases has two main functions, to improve illumination of a feed in a system where the feed is not perfectly matched and to provide a stable and accurate fixing point for the feed.

You will also deduce from your sketch that;

1) A deeper dish has less need of a scalar system than a shallow dish, and
2) A shallow, prime focus dish is less affected by a scalar ring system, as a result of the smaller shadow created.

 

[blademedia]

The object of the scalar rings is to reduce ground noise being seen by the feed horn from beyond the edges of the dish. The rings on the scaler feed are 1/4 wave choke rings which cancel out the signal

 

[John]

Yes J, I was interested in what you get from 3675 now you have the scalar in the correct position.

Optimised the scalar now for what seems to be best --- 3675 R is giving me C/N of between 9.5dB to 10.1 dB on the Promax. A scan on the TBS card / EBSPro read 9.1 dB when it captured it.
A slight fluctuation on received signal on this sat for me.

 

[Channel Hopper]

The object of the scalar rings is to reduce ground noise being seen by the feed horn from beyond the edges of the dish. The rings on the scaler feed are 1/4 wave choke rings which cancel out the signal

Sorry, the scalar feed in all the - prime focus - dishes I have installed is to aid in the alignment of the incoming signals in an imperfect matched system. There is an improvement in overall S/N ratio in almost all cases by under illuminating the dish.

There are secondary improvements in the use of concentric ring system, but from memory the use of accurate mathematics of the wave front directly at the front of the focal point on a parabolic antenna is there to improve the lower frequencies that can be received, offsetting the smaller throat of the feed that raises the centre frequency of the band of interest, waveguide dimensions and the detection system employed.

I will have to go check back through the college notes to be absolutely sure though as I have been out of mechanical reflector designs for a number of years.

 

[blademedia]

I will have to go check back through the college notes to be absolutely sure though as I have been out of mechanical reflector designs for a number of years.

You do that ,

Too big scalar will over illuminate the dish to small will under illuminate the dish, having it set in the wrong place will do similar,

They dont add much to the carrier to noise ratio when in the right position only reduce the noise by about 0.1dB ;)

 

[Channel Hopper]

You do that ,

Too big scalar will over illuminate the dish to small will under illuminate the dish, having it set in the wrong place will do similar,

They dont add much to the carrier to noise ratio when in the right position only reduce the noise by about 0.1dB ;)

0.1 dB on which band ?

A correctly designed feed and matched reflector does not require a scalar ring, so I already know the answer, but will pull the records when I can.

 

[blademedia]

A correctly designed feed and matched reflector does not require a scalar ring

crack on.

 

[John]

Optimised the scalar now for what seems to be best --- 3675 R is giving me C/N of between 9.5dB to 10.1 dB on the Promax. A scan on the TBS card / EBSPro read 9.1 dB when it captured it.
A slight fluctuation on received signal on this sat for me.

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This is from the V7HD fag packet rx, have you any reception pics to share yet

 

[blademedia]

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This is from the V7HD fag packet rx, have you any reception pics to share yet

View attachment 105179

Not at the moment John I have the KU band LNB on, what's concerning me your only matching the 1.8 dish somethings amiss somewhere imho.

 

[moonbase]

If I follow the discussion corectly about the flat scalar on a prime focus dish, there may be no need for a scalar if the C band feed is located at the point where the feed beamwidth angle is converging at the correct position inside the feed barrel? At this position, the feed is located such that ground noise is blocked out by the periphery of the reflector face but the feed is fully illuminated by the dish, is that correct?

Rgds

 

[blademedia]

A scalar is needed to get the best from the dish, it needs to be positioned in the correct place and pointing in the correct spot, it also needs to be the correct one for the dish F/D,

Look at the Seavey feeds, they sell three different ones depending on the F/D of your dish, only difference in the three is the scalar, there would be no point putting a 0.35 0n a 0.30 dish as it would over illuminate the dish and pull in noise.



Look at it like a horse with/without blinkers, without blinkers the horse as a wide field of vision, with blinkers the horse has narrower field of vision. you want the scalar to see the edges of the dish, no more no less to fully illuminate it.

 

[moonbase]

A scalar is needed to get the best from the dish, it needs to be positioned in the correct place and pointing in the correct spot, it also needs to be the correct one for the dish F/D,

Look at the Seavey feeds, they sell three different ones depending on the F/D of your dish, only difference in the three is the scalar, there would be no point putting a 0.35 0n a 0.30 dish as it would over illuminate the dish and pull in noise.
View attachment 105182
Look at it like a horse with/without blinkers, without blinkers the horse as a wide field of vision, with blinkers the horse has narrower field of vision. you want the scalar to see the edges of the dish, no more no less to fully illuminate it.

@blademedia,

Thank you for the explanation. I have another question if you can please help? Is it the distribution/depth of the scalar rings on the scalar or the forward/backward location of the scalar on the feed that matches it to the F/D?

I notice that Chapparal and other vendors place a scale on the side of the feed to indicate adjustment points for different F/D with the same scalar. Presumably this works to some degree but is not the perfect solution?


Rgds